Why did I get a "Permission Denied" error after being logged in for a long time?

If you try to save a file after you've been logged in over 10 hours, you will get a "permission denied" error of some sort. This happens because, after 10 hours, your authentication to Athena expires. In order to be able to use your account again without interruption, all you need to do is reauthenticate yourself. You can do this with the following command:

joeuser@athena:~$ renew

and entering your password when prompted.

After that, you should be able to save files to your home directory again.